Piezoelectric resonator element of cyrstallographic point group 32

ABSTRACT

A piezoelectric resonator element of crystallographic point group  32 , which can be operated as a thickness shear resonator contacting a carrier medium includes a singly rorated Y-cut (S 1 ,S 2 ) that is essentially rotated through an angle Φ about the crystallographic x-axis, which differs from crystal cuts that are temperature-compensated in air or vacuum, wherein the cut has a negative temperature coefficient of the resonance frequency f(T) in a predetermined temperature range, preferably between 10° C. and 40° C., when there is no contact with the carrier medium, while the value of the linear temperature coefficient a of the resonance frequency in the same temperature range is less than 1 ppm/° C., preferably less than 0.5 ppm/° C. when the resonator is in contact with the carrier medium. The resonator element ( 1 ) can additionally be provided with at least one layer sensitive to the parameter to be measured.

[0001] The invention relates to a piezoelectric resonator element of crystallographic point group 32, which can be operated as a thickness shear resonator in contact with a carrier medium, and a measuring system for determining at least one chemical, biochemical or physical parameter in a carrier medium, which carrier medium can be brought into contact, on at least one side, with a piezoelectric resonator element of point group 32 having electrical contacts and—if required—at least one sensitive layer, where at least one resonance property, i.e., preferably the resonance frequency of the resonator element operated as a thickness shear resonator provides a measure for the chemical, biochemical or physical parameter to be determined.

[0002] Such resonator elements are employed for instance in micro-balances, which are based on the effect that the resonance properties of a piezoelectric resonator, usually a thickness shear resonator, are changed by the mass load on the resonator surface and by the viscosity or the electrical conductivity of the adjacent medium. Microbalances of this sort are frequently used for the in-situ measurement of layer thickness via the mass load. In these applications the micro-balances are operated in vacuum. Recently this technique has also been used for determining the concentration of certain components in liquids or gases, where at least one surface of the resonator is provided with a selectively binding layer, which essentially binds only the substance to be measured to the resonator surface, thus increasing the oscillating mass.

[0003] Since the resonance properties of a piezoelectric resonator are not only dependent on the oscillating mass but also on capacitances which act in series with or parallel to the resonator, it is also possible to measure electrical properties such as conductivity or the dielectric constant, respectively changes in these properties, of liquids or gases.

[0004] A disadvantage of all these applications lies in the fact, that temperature changes in the resonator itself as well as in the medium to be measured may exert a considerable influence on the resonance properties, especially on the resonance frequency, which either necessitates the use of costly and cumbersome thermostatic devices to counter the effects of temperature fluctuations, or compels one to accept a reduced sensitivity and accuracy of the sensor. From the paper “Oszillatoren für Quartz-Crystal-Microbalance-Sensoren in Flüssigkeiten”, Technisches Messen 65 (1998) by F. Eichelbaum, R. Borngräber, R. Lucklum, P. Hauptmann and S. Rösler, it is for instance known that the temperature dependence of the resonance frequency, i.e., the oscillator frequency of the predominantly used quartz AT-cut resonators, which is very low in air, especially at room temperature, shows a considerable slope of 35 Hz/° C. at a fundamental frequency of 10 MHz in the case of one-sided contact with water. Stated independently of the fundamental frequency this is 3.5 ppm/° C.

[0005] In U.S. Pat. No. 5,487,981 A for instance, the influence of temperature changes in a liquid on the resonance frequency of a quartz AT thickness shear resonator is measured using a resistance thermometer, and the corresponding offset frequency is computed by a programmable controller and electronically compensated. This of course is a relatively costly procedure.

[0006] The temperature dependence of the resonance frequency of a piezoelectric resonator may for instance be described by the following equation:

[f(T)−f(T ₀)]/f(T ₀)=a(T−T ₀)+b(T−T ₀)² +c(T−T ₀)³

[0007] with

[0008] f(T) resonance frequency at temperature T

[0009] f(T₀) resonance frequency at reference temperature T₀

[0010] a,b,c linear, quadratic, cubic temperature coefficient

[0011] The reference temperature T₀ may be freely chosen. In this patent description only the linear temperature coefficient a is used to describe the temperature dependence of the resonance frequency f(T), which may for instance be determined by a linear fit over a predetermined temperature range, e.g., between 10° C. and 40° C. The quadratic and cubic coefficients are not taken into account.

[0012] It is the object of the present invention to propose a resonator for a measuring system for the determination of a chemical, biochemical or physical parameter in a carrier medium, which avoids the disadvantages mentioned above. Measurement sensitivity is to be preserved or even improved in the case of temperature fluctuations in the resonator or in the medium to be measured without the need for costly correcting devices.

[0013] According to the invention this object is achieved by proposing that the resonator element be a singly rotated Y-cut that is essentially rotated through an angle Φ about the crystallographic x-axis, which differs from crystal cuts that are temperature-compensated in air or vacuum, and that said cut have a negative temperature dependence of the resonance frequency f(T) in a predetermined temperature range, preferably between 10° C. and 40° C., when there is no contact with the carrier medium, while the value of the linear temperature coefficient a of the resonance frequency in the same temperature range is less than 1 ppm/° C., preferably less than 0.5 ppm/° C. when the resonator is in contact with the carrier medium.

[0014] Surprisingly it was found that a piezoelectric resonator, which in air, in a temperature range between 20° C. and 35° C., has a negative linear temperature coefficient a of the resonance frequency f(T)—with a value between 0.5 ppm/° C. and 3.5 ppm/° C., has an almost temperature-independent resonance frequency in this temperature range when in one-sided contact with distilled water.

[0015] The interaction between the oscillating surface of the resonator and the adjacent medium depends largely on the excited oscillation mode. In piezoelectric resonators longitudinal and transversal modes can be excited.

[0016] In the case of a longitudinal mode the direction of the displacement of the crystal surface is essentially normal to the resonator surface. In this case the resonators, especially when in contact with a fluid, show a strong dampening of the resonant oscillation, which leads to severe restriction of the measurement range and the resolution of such sensors.

[0017] Resonators which are excited in essentially transversal oscillation modes show a much better oscillation behavior especially when in contact with liquid media. The best known representatives of this class are so called thickness shear resonators, such as the quartz AT or BT cut.

[0018] The temperature dependence of the resonance frequency of a piezoelectric resonator is determined by the cut-direction of the resonator platelet relative to the crystallographic axes.

[0019] The signs of the cutting angles Φ indicating the rotation direction about the crystallographic axes, are given in accordance with the “IEEE Standard on Piezoelectricity”; ANSI/IEEE Std. 176-1987.

[0020] The quartz industry however still uses the standard from the year 1949, which will give the opposite sign for the cutting angle.

[0021] The following table will show the difference: Standard Quartz AT (Φ[°]) Quartz BT (Φ[°]) 1949 +35°15′ *) −48°50′ *) 1987 −35°15′ +48°50′

[0022] In the case of a preferably aqueous carrier medium with low viscosity (kinematic viscosity γ<15 mm²s⁻¹ at 25° C.), which is in at least one-sided contact with the resonator, the invention proposes the following cutting angles Φ, depending on the application and material chosen for the resonator:

[0023] For quartz in the ranges:

[0024] −36.5°±1.1°, preferably −35.9°±0.5°

[0025] +52.5°±3.0°, preferably +51.4°±1.3°

[0026] For langasite in the range:

[0027] +6.3°±3.0°, preferably +5.3°±2.0°

[0028] For gallium orthophosphate in the range:

[0029] −21.2°±4.5°, preferably −18.5°±1.7°.

[0030] In a variant, where the resonator is in at least one-sided contact with a viscous carrier medium (kinematic viscosity γ>15 mm²s⁻¹ at 25° C.), for instance oil, the following cutting angles Φ are proposed:

[0031] For quartz in the ranges:

[0032] −45.0°±7.0°, preferably −41.5°±3.5°

[0033] +60.0°±8.0°, preferably +60.0°±6.0°

[0034] For langasite in the range:

[0035] +30.0°±19.0°, preferably +22.0°±11.0°

[0036] For gallium orthophosphate in the range:

[0037] −35.0°+10.0°, preferably −40.0°±5.0°.

[0038] The advantages claimed for the resonator according to the invention will largely be maintained if it is ensured during manufacture of the resonator that a rotation about the z-axis preceding the rotation about the x-axis is kept small, values between −10° and +10° , preferably between −5° and +5°, being permissible.

[0039] The invention will be further described below with reference to the enclosed drawings.

[0040]FIG. 1 shows a piezoelectric resonator element according to the invention,

[0041]FIG. 2 shows a measurement system with a piezoelectric resonator element according to the invention in a sectional view,

[0042]FIG. 3 is a schematic view of the measuring system,

[0043]FIG. 4 shows the temperature dependence of the resonance frequency of the resonator element of FIG. 1 in air and in water.

[0044] In FIG. 1 the cutting angles Φ for two quartz resonator elements 1 according to the invention—the two singly rotated Y-cuts S1 and S2—are shown in comparison with AT or BT cuts which are temperature-compensated in air or vacuum, the cuts S1 and S2 being optimised for viscous carrier media such as oil.

[0045]FIG. 2 shows a two-part measurement cell 3 for liquids, whose parts are screw-connected at two points 4. In between there is a piezoelectric resonator element 1 with electrodes at the exterior surfaces, which is sealed against the upper part of the housing by an elastic sealing ring 7. The carrier medium, i.e. the liquid 2 to be measured, is in direct contact with the resonator surface which can be excited to oscillate piezo-electrically. To protect the resonator element 1 against aggressive carrier media it may be coated with a chemically inert protecting layer.

[0046]FIG. 3 is a schematic drawing of a measuring system according to the invention. The piezoelectric resonator element 1 contained in the liquid measuring cell 3 is electrically connected with electronic measurement and evaluation units 5 and 6. The resonator surface may be coated with a layer 8 which is sensitive relative to at least one sample parameter.

[0047]FIG. 4 shows the measured temperature dependence of the resonance frequencies of a quartz thickness shear resonator (Φ=−35.6°), which is provided with gold electrodes on both sides, in air and in one-sided contact with distilled water, in the temperature range between 20° C. and 35° C. While the temperature dependence of the resonance frequency of the resonator oscillating in air shows a negative linear temperature coefficient a with a value of about 2.8 ppm/° C., the linear temperature coefficient a of the resonance frequency of the resonator in one-sided liquid contact is only about 0.14 ppm/° C., i.e. smaller by a factor 20. 

1-5. (Cancel)
 6. A piezoelectric resonator element of crystallographic point group 32, which is operated as a thickness shear resonator in contact with a carrier medium, wherein the resonator element is a singly rotated Y-cut which is essentially rotated through an angle Φ about the crystallographic x-axis, said angle Φ deviating from crystal cuts that are temperature-compensated in air or vacuum, and which has a negative temperature dependence of resonance frequency f(T) in a predetermined temperature range between 10° C. and 40° C., when there is no contact with said carrier medium, while the value of the linear temperature coefficient a of said resonance frequency in said predetermined temperature range is less than 1 ppm/° C. when said piezoelectric resonator is in contact with said carrier medium.
 7. A resonator element according to claim 6, wherein said value of said linear temperature coefficient is less than 0.5 ppm/° C.
 8. A resonator element according to claim 6, which is at least one-sided contact with a carrier medium of low viscosity, wherein the following cutting angles Φ are proposed: for quartz in the ranges: −36.5°±1.1°, and +52.5°±3.0° for langasite in the range +6.3°±3.0° for gallium orthophosphate in the range: −21.2°±4.5°.
 9. A resonator element according to claim 6, which is in at least one-sided contact with a carrier medium of low viscosity, wherein the following cutting angles Φ are proposed: for quartz in the ranges: −35.9° C.±0.5°, and 51.4°±1.3° for langasite in the range: ±5.3°±2.0° for gallium orthophosphate in the range: −18.5°±1.7°.
 10. A resonator according to claim 9, wherein said carrier medium of low viscosity is an aqueous carrier medium.
 11. A resonator element according to claim 6, which is in at least one-sided contact with a viscous carrier medium, for instance oil, wherein the following cutting angles Φ are proposed: for quartz in the ranges: −45.0°±7.0°, and +60.0°±8.0° for langasite in the range +30.0°±19.0° for gallium orthophosphate in the range: −35.0°±10.0°.
 12. A resonator element according to claim 6, which is in at least one-sided contact with a viscous carrier medium, for instance oil, wherein the following cutting angles Φ are proposed: for quartz in the ranges: −41.5°±3.5°, and +60.0°±6.0° for langasite in the range +22.0°±11.0° for gallium orthophosphate in the range: −40.0°+5.0°.
 13. A resonator element according to claim 6, wherein the rotation about the x-axis is preceded by a rotation about the z-axis between −10° and +10°.
 14. A resonator element according to claim 6, wherein the rotation about the x-axis is preceded by a rotation about the z-axis between −5° and +5°.
 15. A measuring system for determining at least one chemical, biochemical or physical parameter in a carrier medium, which carrier medium is brought into contact, on at least one side, with a piezoelectric resonator element of point group 32 having electrical contacts, where the resonance frequency of said resonator element operated as a thickness shear resonator provides a measure for said chemical, biochemical or physical parameter to be determined, wherein said resonator element is a singly rotated Y-cut which is essentially rotated through an angle Φ about the crystallographic x-axis, said angle Φ deviating from crystal cuts that are temperature-compensated in air or vacuum, and which has a negative temperature dependence of resonance frequency f(T) in a predetermined temperature range between 10° C. and 40° C., when there is no contact with said carrier medium, while the value of the linear temperature coefficient a of said resonance frequency in said predetermined temperature range is less than 1 ppm/° C. when said resonator is in contact with said carrier medium.
 16. A resonator element according to claim 15, wherein said piezoelectric resonator element is provided with at least one sensitive layer.
 17. A resonator element according to claim 15, wherein said value of said linear temperature coefficient is less than 0.5 ppm/° C. 